As modern electronic circuit boards evolve toward increased circuit and component densities, thorough cleaning of the boards after soldering becomes more important. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with a flux and thereafter passing this coated side of the board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes adhesion of the solder. Commonly used fluxes consist, for the most part, of rosin used alone, or with activating additives, such as amine hydrochlorides or oxalic acid derivatives.
The temperatures used during soldering usually thermally degrades part of the flux. The remaining flux and flux residues are often removed from the board with an organic solvent. The requirements for such solvents are stringent: a solvent should have a low boiling point, should be nonflammable, have low toxicity and exhibit high solvent power, so that flux and flux residues can be removed without damage to the substrate being cleaned. Flammability and solvent power characteristics can often be adjusted by preparing solvent mixtures, these mixtures are often unsatisfactory because they fractionate to an undesirable degree during use. Such mixtures also fractionate during recovery, making it difficult to recover a solvent mixture with the original composition.
On the other hand, azeotropic mixtures, with their constant boiling points and constant compositions, have been found to be very useful. Azeotropic mixtures exhibit either a maximum or minimum boiling point and do not fractionate on boiling. These characteristics are also important in the use of the solvent compositions to remove solder fluxes and flux residues from printed circuit boards. Preferential evaporation of the more volatile components of solvent mixtures would occur if the mixtures were not azeotropes or azeotrope-like, and would result in mixtures with changed compositions, with attendant less desirable solvency properties, such as lower rosin flux solvency and lower inertness toward the electrical components being cleaned.
The azeotropic character is also desirable in vapor defluxing operations where redistilled solvent is generally employed for final rinse cleaning. Thus, the vapor defluxing and degreasing systems act as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distributions may result to upset the safety and efficacy of the cleaning operation.
A number of chlorofluorocarbon based azeotropic compositions have been discovered and in some cases used as solvents for the removal of solder fluxes and flux residues from printed circuit boards and for miscellaneous degreasing applications. For example, U.S. Pat. No. 3,903,009 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane and ethanol; U.S. Pat. No. 2,999,815 discloses the binary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with acetone; and U.S. Pat. No. 2,999,817 discloses the binary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with methylene chloride. Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes and this fact obviously complicates the search for new azeotropic compositions, which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like compositions which have desirable solvency characteristics and particularly greater versatilities of solvency power.